Acid-resistant enamels



United States Patent 3,383,225 AClD-RESISTA T ENAMELS Norman H.Stradley, St. Paul, Minn., assignor to Minnesota Mining andManufacturing Company, St. Paul, Minn, a corporation of Delaware NoDrawing. Continuation-impart of application Ser. No. 545,537, Nov. 7,1955. This application July 17, 1957, Ser. No. 672,357

14 Claims. (Cl. 106-48) This application is a continuation-in-part of myapplication Ser. No. 545,537, filed Nov. 7, 1955, and now abancloned.

This invention broadly relates to acid-resistant enamels particularlysuitable for aluminum and aluminum alloy base materials. Moreparticularly, this invention is directed to low temperature maturing,high thermal expansion, acid-resistant, as well as alkali-resistant,enamels in the marketable form of porcelain enameling frits.

Compositions within my invention in the form of enamel glasses or fritsare useful for providing crackresistant and stain-resistant enamelcoatings on a variety of metal and allied surfaces, and even may be usedfor coating kitchenware to provide decorative and easily cleanedsurfaces. My enamels possess an unusually high resistance to atta-ck. byacids. In the acid resistance test described hereinafter, my enamelsshow only very low Weight losses, i.e., Weight losses less than about0.06 gram per square inch of an enameled panel. Preferred enamelshereof, which are eminently suitable for providing decorative exteriorcoatings on kitchenware and structural panels, lose less than 0.03 gramper square inch of an enameled panel according to the acid resistancetest described hereinafter. Such low weight losses are indicative of ahighly acid-resistant enamel.

The enamels of this invention retain their original gloss and resistcorrosion over lengthy periods of outdoor weathering. As a comparison,sample enameled aluminum panels of the prior art, exhibiting Weightlosses of about 0.15 to 0.20 gram according to the acid resistance testherein, were found to lose their gloss under outdoor exposure over a fewmonths on a Houston, Tex. exposure rack, whereas similar panels enameledwith the glass frits of this inveniton have effectively retained theirinitial gloss for as long as they have been exposed on the rack, i.e.,almost two years as of the date of executing this present application.

In addition to the exceptional resistance to acid attack exhibited by myenamel compositions, their resistance to alkali attack is noteworthy, aswill be described.

My enamel glass compositions may be fired at temperatures belowapproximately 600 C. In other words, they mature, i.e., fuse, attemperatures below about 600 C. Such low maturing temperatures arenecessary for enamels applied to aluminum surfaces inasmuch as aluminumloses its structural strength when subjected to temperatures somewhatabove 600 C.

Enamels for aluminum also must possess a reasonably high thermalcoefficient of expansion in order.to remain adherent and uncracked onsuch surfaces when subjected to variations in temperature. Enamels madefrom frit compositions hereof have suitable coeificients of thermalexpansion for use on aluminum and, in fact, have thermal coefiicients ofexpansion above 12 10- per C., and as high as about 18x10 per C., overthe range of temperatures to which they are normally subjected in theform of decorative coatings on kitchenware or the like, i.e., from 50350C. My preferred frit compositions have a higher minimum thermalcoeflicient of expansion, i.e., above 14 10- per C. between 50 and 350C.

I have discovered that highly acid-resistant enamels suitable fordecorative use on aluminum type kitchenware can be produced from a novelchemically-resistant alkalititania-silicate system requiring a limitedcontent of oxides of metals selected from Group II of MendelyeevsPeriodic Table. This discovery is my invention. Whilealkalititania-silicate glasses are not new, none known to me other thanthose within my invention possess sufficient resistance to chemicalattack so as to be suitable for various decorative uses on the exteriorof such articles as aluminum cooking utensils or the like. Additionally,my enamels are useful for providing decorative coatings on aluminum-typestructural panels. In such uses, the enamel coating must stand up forlengthy periods under Weathering conditions. Tests indicate that myenamels will satisfy such requirements.

Coatings formed from my enamel frit compositions are especially hard,being harder, insofar as I am aware, than any other enamel designed foraluminum-type surfaces. Using my enamel frits, it is easy to form enamelcoatings of a wide variety of colors, since most inorganic colorpigments have been found to be compatible with the particular enamelcompositions hereof. Also, my particular enamel compositions permit easyformation of white coatings having a high opacity.

Phosphors, e.g. zinc sulfide, may be incorporated in lead-free coatingsformed using my enamel frit compositions to provide coatings of brightfluorescence, Whereas lead oxide in phosphor-containing enamel coatingshas been noted to cause degradation or inhibition of such fluorescingproperties.

I am familiar with British Patent No. 665,903 to Fraser and Cianchi,published Jan. 30, 1952, directed to an enamel for aluminum. This patentpoints out that some titanium dioxide may be included in an enamel, andfurther teaches that rather large amounts of lead oxide and certainbivalent metal oxides are needed in the enamel. Contrary to thisteaching, I have found that lead oxide is an unnecessary constituent inenamels for aluminum, and that it desirably is omitted from thecomposition. In my particular compositions, the various ingredients areso balanced that a small amount of lead oxide added thereto does notenhance the dissolving of titanium dioxide, even though lead oxide is aWell known dissolver for titanium dioxide in certain compositions.

Additionally, I have found that enamels for aluminum surfaces can beformed so as to possess surprisingly high acid resistivity, asaforenoted, by employing only a very small but required amount ofcertain bivalent metal oxides (i.e., Group II oxides as hereinafterdiscussed). These oxides must be present in my frit compositions in onlya critically small amount so that they serve as fluxing aids to dissolvelarge quantities of titanium dioxide into the glassy complex of myenamel frit. Larger amounts of these oxides than herein specified are tobe avoided, since instead of functioning to aid in dissolving titaniumdioxide into the glassy complex, they tend to cause it to beprecipitated therefrom as a component of a crystalline phase, resultingin a loss of the required property of specified high acid resistivityfor a coating. Thus, contrary to expectation,

ritically small amounts of these bivalent metal oxides greatly enhanceacid resistivity apparently by actually dissolving titanium dioxide intothe glassy frit complex and by maintaining most of it in solution, evenduring subsequent maturation of a coating, whereas larger amounts ofthese oxides have been noted to cause titanium dioxide to be largelyprecipitated in crystal form out of the glassy complex of the frit.While I do not wish to be bound by theory, I believe that when thesebivalent metal oxides are present larger amounts than herein specified,they tend to react with titanium dioxide to form a readily precipitatedcrystalline complex, whereas in small amount as hereinafter discussed,they fail to function in this manner and instead function with otheringredients of my frit composition to maintain titanium dioxide incomplex glassy solution. The immediately preceding is the key point ofmy invention in its broadest aspect.

My frit compositions which possess properties satisfying suchrequirements as aforesetforth include a variety of inorganicconstituents within certain defined approximate amounts, andcharacterized by certain general relationships. All percentagerequirements set forth herein are in terms of mol percent of the totalcomposition. My compositions contain from 25 to 50% SiO and from to 25%TiO The ratio of the mol percent of SiO to the mol percent of TiO isbetween 1.4 and 5.0, and the total of SiO and TiO is between 45 and 60%.Included in my compositions is at least one TiO -dissolving oxideselected from the group consisting of CdO, ZnO, 82.0, 810, C210, andMgO. The maximum amount of any one TiO -dissolving oxide in mycompositions does not exceed about 8%, with the exception that CdO maybe present up to 10%; and the total amount of the TiO -dissolving oxidesshould be maintained between about 2 and 10%. The total mol percentamount of TiO and TiO -dissolving oxides in my compositions ismaintained below the mol percent amount of SiO therein. Alkali metaloxides are required approximately as follows: from 5 to of Li O, from 2to 13% of K 0, and from 15 to Na O, the total of Li O, K 0 and Na Obeing from 25 to B 0 is maintained between 0.5 and 12%, while P 0 mayvary from 0 to 5%. The total B 0 and any P 0 varies from 1 to 12%. Theforegoing enumerated constituents make up at least about 90 mol percentof the total frit composition. Small amounts of inorganic oxides otherthan those aforementioned may also be present in my frits withoutdeparting from the essential nature of the invention hereof, a principlewellrecognized in the porcelain enamel art.

The various proportions and the relationship of the various essentialconstituents in the glass system of my enamels are considered criticalin that proportions of constituents outside the aforementionedapproximate ranges and a relationship of constituents not satisfying theforegoing criteria result in a loss of one or more of the propertiesaforesetforth for the aluminum enamels hereof. For example, an excess ofSiO (i.e., an amount in excess of the amount specified) raises thematuring temperature while a deficiency causes a loss of aciddurability. TiO greatly improves acid durability and surprisingly alsocauses a lowering of the maturing temperature of the enamel glass orfrit but gives such a result only when present in the approximate amountand relationship as above specified. The mol percent amount of TiO plusthe total mol percent amount of oxides of metals selected from Group IIof Mendelyeevs Periodic Table (i.e., CdO, ZnO, BaO, SrO, CaO, and MgO)must not exceed the total mol percent amount of SiO in the composition.An excess of Group II metal oxides above the approximate amountspecified causes a substantial decrease in acid durability.

For its high =fiuxing properties combined with its property of impartingacid resistance to my compositions, Li O must be maintained within theapproximate mol percent amounts specified. Larger amounts cause a lossof low temperature maturing properties. K 0 is necessary for highthermal expansion but an excess above the amount specified pronouncedlylowers the acid durability of the enamel. Na O is used with the otheralkali metal oxides to improve fiuxing and working characteristics ofthe enamel glass as well as to offset the tendency toward loss indurability caused by the rather high amounts of K 0 re quired in myenamels for expansion properties. The total required alkali metal oxidecontent may be noted to be rather high in my enamels and, admittedly, isa rather surprising requirement for highly acid resistant enamels.

A small amount of the low ten perature glassformer, B 0 is necessary fordesired maturing properties and glass formation, but an excess aboveabout 12 mol percent greatly lowers durability. P 0 is an optionalconstituent, and if present, it will not exceed about 5 mol percent ofthe total composition. While P 0 is a well-known conventional lowtemperature glassformer, larger amounts than 5 mol percent in mycompositions tend to cause a loss of gloss and of low temperaturematuring properties in the inorganic system employed.

F is not required constituent, but may be used in my compositions forfluxing purposes, if desired, in an amount no greater than 5 molpercent. It is usually added as NaF, in which ease up to 10 mol percentof NaF (which is the equivalent of 5 mol percent of F may be employedfor the fluorine content. However, other alkali or alkaline earthfluorides in amounts giving an equivalent fluorine content may be used.

My preferred frit compositions have maturing temperatures belowapproximately 560 C., coeflicients of thermal expansion above 14 l0- perC. between and 350 C., and acid resistivities so great that they exhibitonly a small weight loss of less than about 0.03 gram per square inchaccording to the acid test herein. They are in part characterized by 2.mol percent range for various constituents and proportion relationshipbased on total composition as follows: from 30 to 45% SiO from 12 to 22%TiO the ratio of Si0 to TiO being from 1.8 to 3.5 and the total of Si0and TiO being between 48 and 60%, at least one TiO -dissolving oxide, inthe indicated percentage range, selected from the group consisting of upto 10% CdO, up to 8% ZnO, up to 8% Eat), up to 8% SrO, up to 8% CaO, andup to 8% MgO, the total of said TiO -dissolving oxides being between 2and 10%, the total mol percent amount of TiO and TiO -dissolving oxidesbeing less than the mol percent amount of SiO and the ratio of the sumof the mol percent of SiO and Ti9 over the total mol percent ofTlOgdissolving oxides being at least 6, from 6 to 13% of Li O, from 4 to12% of K 0, from 16 to 22% of Na O, the total of Li O, K 0 and Na Obeing between 30 and 40%, from 1 to 7% of B 0 and from 0 to 3.5% P 0 thetotal of B 0 and any P 0 being from 2 to 8%, the total of the foregoingconstituents accounting for at least about 90% of the composition. Asnoted above, F may be present in these compositions, if desired, in anamount up to 5 mol percent.

Additionally, in one embodiment of my invention directed to preferredenamels having extraordinarily high acid resistivities, a small amountof 513 0 up to about 1 mol percent is present. I have found that Sb Oamong other things as will be explained, functions apparently to inhibitcrystal growth and precipitation of crystals from my glassy enamels; andin this manner it serves to increase the acid resistance exhibited bythe enamel. For example, composition number 30 in Table III is similarto composition number 26 except for a small amount of Sb O whichsurprisingly increases the acid resistivity of the enamel composition.

In another embodiment of my invention directed to preferred enamelshaving extraordinarily high acid resistivities, at least two TiO-dissolving oxides are employed. They seem to behave synergistically inthat when two or more are present, and the maximum amount of any one TiO-dissolving oxide is not in excess of 5%, glass enamel frits of thisinvention show improved acid resistivity as well as generally improvedworking characteristics during application and maturation of the enamelas a film coating. CdO in addition to promoting the solubility of TiO inmy glass frits as noted above, also promotes excellent adhesion of theresulting enamel to aluminum and stabilizes cadmium sulphoselenidepigments in the enamel so that bright red and yellow glossy enamelcoatings can be formed.

Whenever CdO is omitted from compositions formed as set forth above, theresulting enamel coating exhibits somewhat less tenacious adherence tometal, particularly aluminum, than is satisfactory under most conditionsof use. To gain satisfactory metal adherence, I may employ any adherencepromoting oxide, or mixtures thereof, up to the total amount of 5 molpercent of the total composition. CdO, performing a distinctly differentprimary function (i.e., that of dissolving TiO in my compositions, mayadditionally be present within the percent amount specified for thatoxide in my compositions independently of the presence or absence of anyother adherence promoters. Suitable adherence promoters are CuO, NiO,CoO, Sb O PbO, Bi O and mixtures thereof. With the exception of Sb Owhich like CdO also performs another desired function in my fritcompositions, these various adherence promoters may be employed inamounts up to 5 mol percent. Not more than about 1 mol percent of Sb Oshould be employed because this oxide causes a loss of gloss in theresulting enamel. For this reason Sb O is preferably, but notnecessarily, employed with small amounts of another adherence promoteror with CdO present. By varying the amounts of C00, CuO and NiO in thefrit, various colors from deep-blue to blue to blue-green can beobtained in the frit and thus in the enamel coating. Sb O PhD and Bi Oas adherence promoters permit the formation of colorless coats. PbO, ofcourse, is generally not preferred as a constituent because it is atoxic material.

To prepare my glass enamel frits, raw batch materials, e.g., CaCO CaFTiO etc., mixed in amounts calculated to give the mol percent analysishereinbefore discussed, are melted together at a temperature range ofabout 1000" C. to 1200 C., and then quenched in water and dried. Forexample, a preferred frit composition having the analysis of Example 10may be prepared by mixing 15.6 parts by weight of TiO 1 part ZnO, 4.6parts BaCO 3 parts CdO, 6.7 parts Li CO 7.7 parts of Na P O parts Na COparts K2C03, 1 Part NaF, 22.7 parts SiO and 6.4 parts H BO Afterthoroughly mixing these materials, they are melted in a refractoryvessel, such as an alumina-silicate vessel, at a furnace temperature ofapproximately 1100 C. The mass is heated until it melts down to ahomogeneous fluid state, and then is quenched in water, which actionshatters the glass into small particles. The frit is then dried and isready for use in a slip formulation for enameling.

Slip formulations may be prepared by combining the frit hereof withwater and suitable suspending and/or binding agents, i.e., sodiumsilicate, borax, etc., and then grinding the mixture to suitablefineness for smooth application to clean surfaces. To the slip formulamay also be added coloring oxides, e.g., cobalt aluminate blue,

nickel chromate green, etc., up to an amount of about 10% by weight ofthe slip formulation, if desired. Also, if desired, opacifying oxidessuch as SnO ZrO TiO etc., may be additionally added up to an amount ofabout 15% by weight of the mill or slip formula. Small amounts of milladditives such as potassium dichromate may be desirably used to improvethe surface appearance of fired enamel coatings, i.e., to providecoatings essentially free of pinholes and of improved tear and chipresistance. Slip formulations vary greatly and by using differentformulations it is possible to vary somewhat the properties of aresulting enamel. One illustrative slip formulation is as follows: 100parts of frit, 3.36 parts of potassium silicate powder (28.3% K and70.7% SiO 2.12 parts of KOH, 2.53 parts of H BO and 50 cc. of water.Another is: 100 parts of frit, 9.1 parts of sodium silicate solution(8.9% Na O, 28.7% SiO and 62.4% water), parts of K2B4O7'5H2O, 1.8 partsof KOH, and 50 cc. of water. A third is: 100 parts of frit, 4 parts of awatersoluble crushed glass mixture formed of 9.3% Na O, 34.7% K 0, 25.5%B 0 and 30.5% SiO and 50 cc. of water. All three of these aforenotedslip formulations have been employed with my enamels with verysuccessful results.

The following merely illustrates a further preferred slip formulation,as well as a suitable procedure for enameling: grams of the frit ofExample 10 were mixed in a ball mill with 2 grams of anhydrous borax, 1gram of KOH, 3 grams of sodium meta-silicate (21.8% Na O, 21.1% SiO57.1% H 0), and .50 cc. of water. (This particular slip formulation wasused in making all specimen enameled panels for testing according to theacid resistance and alkali resistance tests described below.) Themixture was ground until the residue, after screening on a 325 meshscreen, was less than 1 gram per 50 cc. of slip. An aluminum alloy panelwas cleaned by washing with carbon tetrachloride, dried, and sprayedwith a sufficient amount of the slip to give a coating weight of 0.3gram of enamel per square inch. Other methods of applying the slip tothe panel are also suitable, methods such as dipping and electrostaticspraying included. The coated panel was then air dried to remove Waterand the piece fired at 520 C. for 10 minutes, resulting in the formationof a porcelain enamel coating free of surface defects such as forexample pinholes, and possessing a high gloss, a high degree ofadherence and a white semi-opaque color.

Aluminum alloy panels that contain large amounts of silicon, magnesium,etc., in the alloy are sometimes diflicult to enamel unless a suitableoxide surface for enameling is first prepared. A suitable preliminarytreating procedure for such panels is as follows: first clean organicmatter from the panel "by a soak period of about 3 to 4 minutes intrichloroethylene. Then dip the panel for about 2 minutes in a 5% sodiumhydroxide solution held at about 180 F. Next rinse the panel thoroughlywith water and dip it for about 2 minutes in a 30% nitric acid solutionheld at about 180 F. Following this, again rinse the panel in hot waterand immerse it for about 10 minues in boiling water. Then dip it forabout 2 minutes in a 5% potassium chromate solution held at about 180 F.or higher. Finally rinse the panel thoroughly and allow it to dry. Theresulting cleaned panel may be enameled Without difficulty according towell known procedures such as described above.

While primarily designed for application to aluminumtype surfaces, myenamels are also useful in coating such material as copper and silver.The relatively high dielectric constant of my enamel may render itparticularly useful in the electrical industry.

In Tables I, II and III, various illustrative examples of this inventionare set forth in terms of the mol percent analysis of inorganicconstituents as calculated from the raw batch. Examples 5 through 10,inclusive, 12 through 24 inclusive, and 30, illustrate preferred fritcompositions hereof.

In the row labeled Acid test are set forth the measured weight lossesresulting from an acid resistance test which involves refluxing 25 ml.of boiling (100 C.) 6% citric acid (by weight) in contact with a 1"square area of an enameled panel having a coating weight of enamel ofabout .3 gram per square inch (which weight is well above the averagecoating weight employed commercially) for 2% hours. (Of course, a largerarea and proportionately larger quantity of 6% citric acid, etc., may beused in conducting the test; and thereafter, appropriate calculationmade to reduce results to the standard herein set forth.) Specifically,in conducing the test, an enameled panel .of adequate size is placed,enamel face upward, on a steel plate over a heating element. Over theenamel coating is rigged a bell jar (a jar having a small throat openingabove and having its bottom removed) having a one square inch total openbottom area. A sealing gasket also having an opening of one square inchin total area is interposed between the enamel surface and the bottomedge .of the bell jar. 25 ml. of hot 6% citric acid solution is thencharged into the bell jar, a reflux condenser unit fitted over thethroat opening of the jar, and boiling of the citric acid solution at100 C. continued for 2% hours. The enameled panel is weighed before andafter the boiling acid treatment and the weight loss caused by thetreatment is a rather accurate indication of what acid resistance theenamel coating will exhibit in practical uses. It also is a ratheraccurate indication of the weather resistance of the enamel.

In order for an enamel to be suitable for decorative square enameledspecimen panel, using the technique otherwise described for the acidresistance test above. My enamels all exhibit a high alkali resistance,according to this test, and in fact, exhibit weight losses less thanexterior use on kitchenware, it should have an acid resist- 5 about 0.05gram per square inch of enameled panel. For ance, as measured accordingto the foregoing boiling citric preferred compositions, the alkaliweight loss is less than acid test, exhibiting only a low weight loss ofnot more about 0.03 gram per square inch of panel. This high alkali thanabout 0.03 gram per square inch. Por elain enamel resistance isparticularly noteworthy when it is found in coats made using thepreferred enamel frit compositions Combination With a g acid fcsistance,as it is in the hereof can satisfy this requirement, and in addition,satisfy 10 Cas Of the enamels taught herein. other requirements foraluminum-type enamels, To my The procedures fOllOWed fOI the Acid testand Alkali knowledge no prior art alkali-titani ili t e el i test hereinset forth give results substantially the same suitable for suchdecorative use on aluminum type kitchen- 35 those Obtained UsingProcedures Outlined i m ware. None known to me possesses sufiicientresistance to Cifil Standard CS10O-47, recommended indus ry in acidattack in combination with other required properties. 15 June of 1956 bythe Quality Development Sub-Commit- The ow l b led Alkali te t o tai thweight losses tee of the Aluminum Division, Porcelain Enamel Institute,of the enamels according to an alkali resistance test com- IIIC- 35 i1tentative Standard for evaluating the q y of prising refluxing 25 ml. ofa boiling (100 C.) solution Porcelain enamel n u n of hydrated odiumpyrophosphate Approximate maturing temperatures in degrees cen- 20tigrade and coeflicients of thermal expansion per C. (Na P O -H O) from50 to 350 C. are set forth for each glass frit.

Additionally certain ratios between ingredients for each by weight, incontact for 2 /2 hours with a one inch composition are set forth in thetable TABLE I Ex.No 1 2 3 4 5 6 7 s s 10 Total 6 6 6 6 5.4 3.0 2 6.2 5.57. 5

Ratio SiO /TiO 1.62 1.88 1.88 1.88 3.31 2.38 1.88 1.94 3.31 1.9: RatioTiO /Total Group II 2.33 1.78 1.78 1.78 1. 61 2.19 2.33 3. 69 2.17 3. 4Ratio T10z+S10z/T0tal Group II. 6.11 5.11 5.11 5.11 6. 94 7.41 5.5710.86 9.3 10.0 Acid test 037 047 031 033 009 018 027 021 007 024 Alkalitest .034 .048 034 036 029 016 014 020 010 .025 a 560 530 550 540 560540 560 540 560 520 l) 15.0 16 3 15.1 14.8 14.3 14.6 15.1 15.4 14.7 16.1

See footnotes at end of Table III.

TABLE II Ex. No 11 12 13 14 15 16 17 18 19 20 TABLE IICntinned Ex. N0 1112 13 14 14 16 17 18 19 20 Ratio SiOz/TiOz 1.94 3.31 1. 94 1.95 1. 941.95 2.25 3. 32 Ratio TiOz/Total Group II 6 39 6 4 5. 53 2.82 5. 53 5.58 5. 53 5. 58 3.95 1. 97 Ratio TiO2+SiOz/Total Group IL 12.2 16.3 16. 1.3 16.5 8.51 Acid test .010 .021. Li. .0 1 .01

Alkali test 023 Total 1.6 3.4 1.6 3.6 .4 .4

Ratio Slog/T102 v 3. 31 1. 94 1. 95 4. 93 2. 46 2. 55 2. 46 2. 46 2. 16Ratio 'llOz/Total Group II 5. 53 2. 83 57 53 5. 58 1.11 2.10 1.41 2.102.10 2. 0S Ratio TiOz-l-SiOg/Totui Group 16. 3 12. 2 16. 3 16. 5 6. 597. 28 5. 01 7. 2S 7. 28 7. 24 Acid test .22 008 024 028 059 039 035 0630i 3 028 Alkali test.. 019 022 000 013 020 012 038 030 038 031 a 540 550540 540 580 540 540 540 510 5 .0 b 16. 2 14. 5 16. 6 10.4 12. 6 16. 516.4 17. 5 17. 0 16. 1

a=Maturing Temperature in degrees Centigrade.

b=Coetficient of thermal expansion per 0. between 50 and 350 C. (Valuesset; forth are times actual values.)

That which is claimed is:

1. A frit particularly adapted for application to surfaces of thealuminum type, characterized by being matnrable into a smooth uniformenamel coating at temperatures below approximately 600 C., exhibiting acoeflicient of thermal expansion above 12 l0 per C. between 50 and 350C., and possessing a high resistance to acid attack, as determined bythe acid test herein defined, giving a weight loss of less than about0.06 gram per square inch of an enameled panel, said frit comprising, interms of essential oxide constituents set forth in mol percent of thetotal composition, from 25 to 50% SiO from 10 to 25% TiO the ratio ofS10 to TiO being from 1.4 to 5 and the total of SiO and Ti0 beingbetween 45 and 60%, at least one TiO -dissolving oxide, in the indicatedpercentage range, selected from the group consisting of 0 to 10% CdO, 0to 8% ZnO, 0 to 8% BaO, 0 to 8% SrO, 0 to 8% C210, and 0 to 8% MgO, thetotal of said TiO -dissolving oxides being between 2 and 10%, the totalmol percent amount of TiO and TiO -dissolving oxides being less than themol percent amount of SiO from 5 to of M 0, from 2 to 13% of K 0, from15 to Na O, the total of M 0, K 0 and Na O being from 25 to from 0.5 to12% of B 0 from 0 to 5% of P 0 the total of B 0 and any P 0 being from 1to 12%, and the mol percent of TiO in the said composition being greaterthan the mol percent of B 0 therein, the foregoing enumeratedconstituents totalling at least about 90 mol percent of the frit.

2. The frit of claim 1 containing fluorine in an amount up to theequivalent of 5 mol percent of F 3. The frit of claim 1 containing inaddition at least one adherence promoting oxide in the approximateindicated mol percent proportion selected from the group consisting of 0to 5% CuO, 0 to 5% NiO, 0 to 5% C00, 0 to 1% Sb O 0 to 5% PbO and 0 to5% Bi O the total amount of said adherence promoting group being up toapproximately 5 mol percent of the frit.

4. A frit particularly adapted for application to surfaces of thealuminum type, characterized by being maturable into a smooth uniformenamel coating at temperatures below approximately 560 C., exhibiting acoefficient of thermal expansion above 14x10 per C. between 50 and 350C., and possessing a high resistance to acid attack, as determined bythe acid test herein defined, giving a weight loss of less than about0.03 gram per square inch of an enameled panel, said frit comprising, interms of essential oxide constituents set forth in mol percent of thetotal composition, from 30 to 45% SiO from 12 to 22% TiO the ratio ofSiO- to Ti0 being from 1.8 to 3.5 and the total of Si0 and TiO beingbetween 48 and 60%, at least one TiO -dissolving oxide selected from thegroup consisting of 0 to 10% CdO, 0 to 8% ZnO, 0 to 8% 13210, 0 to 8%S10, 0 to 8% CaO, and 0 to 8% MgO, the total of said TiO -dissolvingoxides being between 2 and 10%, the total mol percent amount of TiO andTiO -dissolving oxides being less than the mol percent amount of SiO andthe ratio of the sum of the mol percent of SiO and TiO over the molpercent of TiO -dissolving oxides being at least 6, from 6 to 13% of LiO, from 4 to 12% of K 0, from 16 to 22% of Na O, the total of Li O, K 0,and Na O being between 30 and 40%, from 1 to 7% B 0 from 0 to 3.5% P 0the total of B 0 and any P 0 being 1 1 from 2 to 8%, the foregoingenumerated constituents totalling at least about 90 mol percent of thefrit, and a small amount of Sb O up to about 1 mol percent of said frit.

5. A frit particularly adapted for application to surfaces of thealuminum type, characterized by being maturable into a smooth uniformenamel coating at temperatures below approximately 560 C., exhibiting acoefficient of thermal expansion above 14X l per C. between 50 and 350C., and possessing a high resistance to acid attack, as determined bythe acid test herein defined, giving a weight loss of less than about0.03 gram per square inch of an enameled panel, said frit comprising, interms of essential oxide constituents set forth in mol percent of thetotal composition, from 30 to 45% SiO from 12 to 22% TiO the ratio ofSiO to TiO being from 1.8 to 3.5 and the total of SiO and TiO beingbetween 48 and 60%, at least two TiO -disso1ving oxides selected fromthe group consisting of CdO, ZnO, BaO, SrO, CaO, and MgO, the maximumamount of any one TiO -dissolving oxide not exceeding 5% and the totalof said TiO -dissolving oxides being between 2 and the total mol percentamount of Ti0 and TiO -dissolving oxides being less than the mol percentamount of SiO and the ratio of the sum of the mol percent of SiO and TiOover the mol percent of TiO -dissolving oxides being at least 6, from 6to 13% of Li O, from 4 to 12% of K 0, from 16 to 22% of Na O, the totalof Li O, K 0, and Na O being between 30 and 40%, from 1 to 7% B203, frOm0 to P205, the tQtal Of B203 and any P 0 being from 2 to 8%, theforegoing enumerated constituents totalling at least about 90 molpercent of the frit.

6. The frit of claim 5 containing fluorine in an amount up to theequivalent of 5 mol percent of F 7. The frit of claim 5 containing inaddition at least one adherence promoting oxide in the approximateindicated mol percent proportion selected from the group consisting of 0to 5% CuO, 0 to 5% NiO, 0 to 5% C00, 0 to 1% Sb O 0 to 5% PbO and 0 to5% Bi O the total amount of said adherence promoting group being up to 5mol percent of the frit.

8. As a new enameled article of manufacture: a base structure having ametal surface high in aluminum content and a fused acid-resistantporcelain enamel coating adhered to said base structure, said enamelbeing formed from a frit having the composition of claim 1.

9. As a new enameled article of manufacture: a base structure having ametal surface high in aluminum content and a fused acid-resistantporcelain enamel coating adhered to said base structure, said enamelbeing formed from a frit having the composition of claim 2.

10. As a new enameled article of manufacture: a base structure having ametal surface high in aluminum content and a fused acid-resistantporcelain enamel coating adhered to said base structure, said enamelbeing formed from a frit having the composition of claim 3.

11. As a new enameled article of manufacture: a base structure having ametal surface high in aluminum content and a fused acid-resistantporcelain enamel coating adhered to said base structure, said enamelbeing formed from a frit having the composition of claim 4.

12. As a new enameled article of manufacture: a base structure having ametal surface high in aluminum content and a fused acid-resistantporcelain enamel coating adhered to said base structure, said enamelbeing formed from a frit having the composition of claim 5.

13. As a new enameled article of manufacture: a base structure having ametal surface high in aluminum content and a fused acid-resistantporcelain enamel coating adhered to said base structure, said enamelbeing formed from a frit having the composition of claim 6.

14. As a new enameled article of manufacture: a base structure having ametal surface high in aluminum content and a fused acid-resistantporcelain enamel coating adhered to said base structure, said enamelbeing formed from a frit having the composition of claim 7.

References Cited UNITED STATES PATENTS 2,909,438 10/1959 Kantz 106-482,842,458 7/1958 Feeney et al. 106-48 2,414,633 1/1947 Bryant 106-482,492,523 12/ 1949 Cofi'een et al. 106-49 2,604,410 7/1952 Bryant 106482,660,531 11/1953 Fraser et al. 106-48 2,662,020 12/ 1953 Schofield etal. 106-48 2,911,312 11/1959 Hoffman 106-48 FOREIGN PATENTS 665,9031/1952 Great Britain.

HELEN M. MCCARTHY, Primary Examiner.

JOHN R. SPECK, Examiner.

D. J. ARNOLD, R. E. JONES, Assistant Examiners.

1. A FRIT PARTICULARLY ADAPTED FOR APPLICATION TO SURFACES OF THEALUMINUM TYPE, CHARACTERIZED BY BEING MATURABLE INTO A SMOOTH UNIFORMENAMEL COATING AT TEMPERATURES BELOW APPROXIMATELY 600*C, EXHIBITING ACOEFFICIENT OF THERMAL EXPANSION ABOVE 12X10**6 PER *C. BETWEEN 50 AND350*C., AND POSSESSING A HIGH RESISTANCE TO ACID ATTACK, AS DETERMINEDBY THE ACID TEST HEREIN DEFINED, GIVING A WEIGHT LOSS OF LESS THAN ABOUT0.06 GRAM PER SQUARE INCH OF AN ENAMELED PANEL, SAID FRIT COMPRISING, INTERMS OF ESSENTIAL OXIDE CONSTITUENTS SET FORTH IN MOL PERCENT OF THETOTAL COMPOSITION, FROM 25 TO 50% SIO2, FROM 10 TO 25% TIO2, THE RATIOOF SIO2 TO TIO2 BEING FROM 1.4 TO 5 AND THE TOTAL OF SIO2 AND TIO2 BEINGBETWEEN 45 AND 60%, AT LEAST ONE TIO2-DISSOLVING OXIDE, IN THE INDICATEDPERCENTAGE RANGE, SELECTED FROM THE GROUP CONSISTING OF 0 TO 10% CDO, 0TO 8% ZNO, 0 TO 8% BAO, 0 TO 8% SRO, 0 TO 8% CAO, AND 0 TO 8% MGO, THETOTAL OF SAID TIO2-DISSOLVING OXIDES BEING BETWEEN 2 AND 10%, THE TOTALMOL PERCENT AMOUNT OF TIO2 AND TIO2-DISSOLVING OXIDES BEING LESS THANTHE MOL PERCENT AMOUNT OF SIO2, FROM 5 TO 15% OF LI2O, FROM 2 TO 13% OFK2O, FROM 15 TO 25% NA2O, THE TOTAL OF LI2O, K2O AND NA2O BEING FROM 25TO 40%, FROM 0.5 TO 12% OF B2O3, FROM 0 TO 5% OF P2O5, THE TOTAL OF B2O3AND ANY P2O5 BEING FROM 1 TO 12%, AND THE MOL PERCENT OF TIO2 IN THESAID COMPOSITION BEING GREATER THAN THE MOL PERCENT OF B2O3 THEREIN, THEFOREGOING ENUMERATED CONSTITUENTS TOTALLING AT LEAST ABOUT 90 MOLPERCENT OF THE FRIT.